Method for analyzing images and for correcting the values of video signals

ABSTRACT

In a method for image analysis and correction of video signal values in the processing of video films, whereby analysis values with respect to image range, gradation and color cast of the individual frames of the video film are identified by the image analysis of the video signal values, and the correction of the video signal values is implemented on the basis of the analysis values, the video signal values are initially transformed according to a three-dimensional conversion function into the video signal values of an independent and sensation-conforming, equidistant color space. For each frame of a video sequence composed of a plurality of frames, at least one analysis value is determined on the basis of the transformed video signal values. At least one analysis values representative of the video sequence is formed by forming the average of the corresponding analysis values of the frames of the video sequence.

BACKGROUND OF THE INVENTION

The invention is in the field of electronic video technology and isdirected to a method for image analysis and correction of video signalvalues in the processing of video films.

In the production of a video film, individual scenes are recorded with avideo camera, whereby each scene represents a video sequence composed ofa plurality of frames, and the video signal values are deposited in animage store. After the shots, the video film is processed in a videoprocessing station. In the post-processing, the design of the video filmoccurs by cutting and by producing titles as well as a color correctionsupported by an image analysis with which deficiencies are compensatedor colors are editorially changed.

In an image analysis, color values are investigated, for example, inview of the image range, the image gradation and a potentially existingcolor cast. Image light and image dark values are acquired with an imagerange analysis for an adaptation of the image range; a gradationcorrection curve for a contrast correction is determined with an imagegradation analysis; and color cast values for a color cast correctionare determined with a color cast analysis.

For the traditional color correction, analog image analysis methodsbased on the color values R, G and B are usually employed wherein theimage range analysis and the color cast analysis occur directly on thebasis of the color values R, G and B, and the image range analysisoccurs on the basis of a brightness signal derived from color values R,G and B. The traditional methods have the disadvantage that they arerelatively slow and that the analysis results must be largely manuallyconverted into corresponding corrections.

DE-C-43 10 727 already discloses a method for the analysis of an imageoriginal to be reproduced in the preliminary printing stage, whereby RGBcolor values are transformed into corresponding Lab color values of asensation-conforming, equidistant CIELAB color space, and the analysisof the image original is undertaken on the basis of the transformed Labcolor values, as a result whereof optimum analysis results are nearlyautomatically obtained in view of image range, image gradation and/orcolor cast. This publication, however, contains no teaching to apply theanalysis method to the video signal values of individual frames andvideo sequences of a video film.

EP-A-0 723 363 discloses a method for realizing an inventive coloradaptation, whereby RGB color values of individual color images or colorimage sequences are transformed into a color space adapted to the visualsensation of man and are subsequently transformed back. Although theknown method is applied to color image sequences, no analysis withrespect to image range, image gradation and/or color cast occurs on thebasis of transformed Lab color values of the CIELAB color space.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to improve a methodfor image analysis and correction of video signal values in theprocessing of video films such that optimum analysis results areachieved for a color correction of video frames and video sequences.

According to the present invention, a method is provided for imageanalysis and correction of video signal values and processing of a videofilm composed of video sequences, each video sequence being composed ofa plurality of frames. Video signal values of a first color space aretransformed according to a three-dimensional conversion function intovideo signal values of a second color space that is independent of thefirst color space and is equidistant and conforming to sensation. Atleast one analysis value representative of a video sequence is formed byforming an average of analysis values of analyzed frames of the videosequence. A three-dimensional correction function is respectivelydetermined from the analysis values representative of the videosequence. At least the video signal values of the corresponding videosequence are corrected according to the identified correction function.

The invention is explained in greater detail below on the basis of theFIGURE.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows in a block diagram the fundamental signal flow inconjunction with a video processing station.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A video camera 1 as an input device registers a plurality of videosequences composed of individual frames. The video signal values R, Gand B generated by the video camera 1 are deposited in an input imagestore 2. After the exposure, the video film is processed in a videoprocessing station 3. In the post-processing, the design of the videofilm initially occurs by cutting and by producing titles in a cuttingstation 4. For that purpose, the video signal values (R, G, and B) aretransferred from the input image store 2 into the cutting station 4 andare written back onto the input image store 2 after the processing.

After the design, the color correction supported by an image analysisoccurs, deficiencies being compensated or colors being editoriallymodified therewith. For that purpose, the video signal values R, G and Bare first supplied to an input color converter 5 wherein the videosignal values R, G and B of the input color space specified for thevideo camera 1 are transformed according to the three-dimensionalconversion function L, a, b=f(R, G, B) into the video signal values L, aand b of the sensation-conforming, equidistant L, a, and b color spacethat is independent from the input color space.

The transformed signal values L, a and b are supplied video by video toan image analysis unit 6 at which a pre-selection can be undertaken asto whether the image analysis is to occur with respect to the imagerange and/or the image gradation and/or a potentially existing colorcast.

In the image range analysis of a frame, the frequency distribution(brightness histogram) of the brightness component L of the transformedvideo signal values L, a and b in the frame is identified. Subsequently,the image light value B_(L) of the frame and the image dark value B_(T)of the frame are determined from the frequency distribution by definingthe maximum and minimum brightness value L dependent on the curve of thefrequency distribution in the image light region and in the image darkregion. The image range analysis is disclosed in detail in DE-C-43 10727 and DE-C-43 09 879.

In the image gradation analysis of a frame, the frequency distributionof the brightness values L is advantageously defined only from theimage-important regions of a frame. For that purpose, the frame isinitially divided into sub-frames. For each sub-frame, a histogram ofthe brightness values L in the form of sub-image histograms isdetermined. The sub-image histograms are interpreted with statisticalmethods and the image-important sub-frames are classified. From thesub-image histograms of the classified sub-frames, an aggregatehistogram is calculated that corresponds to the frequency distributionof the brightness values L from the image-important regions of theframe. A correction curve for the image gradation characteristic is thenderived from the aggregate histogram according to a method of histogrammodification. The image gradation analysis is disclosed in detail inDE-C-43 10 727 and DE-C-43 09 878.

In color cast analysis, the region of high brightness values and/or theregion of low brightness of the brightness component L in the frame tobe analyzed is divided into at least one brightness region for reliableacquisition of a color cast in the frame. The color cast in therespective brightness range is determined as an average deviation of thevideo signal values from non-chromatic pixel-by-pixel averaging of thevideo signal values a and b in the corresponding brightness regions. Thecolor cast to be consulted for the interpretation of the color castanalysis is then formed by selection and/or combination of color castsdetermined for the brightness ranges. The color cast analysis isdisclosed in detail in DE-C-43 10 727 and DE-C-43 09 877.

Inventively, a set of analysis values such as image light value, imagedark value, color cast value and gradation correction values areinitially determined in the image analysis for each of the m frames ofthe respectively current video sequence, and the set of analysis valuesthat is representative of the current video sequence is then determinedby forming the average from the m sets of analysis values of the frames.

In order to obtain faster analysis results, it proves advantageous toanalyze only every n^(th) frame of a current video sequence, and todetermine the set of analysis values representative of the current videosequence from the n sets of analysis values of the investigated frames,whereby “n” can be selected by the operator.

For further improvement of the results of the image analysis, amodification of the averaging of the image range advantageously occursin that a determination is made on the basis of an empiricallyidentified weighting factor G as to whether the actual image light valueB_(L) and image dark value B_(T) deviates from the averaged image lightvalue B_(LM) and image dark value B_(TM), so that the correspondingcorrection has a somewhat more or less effect. The modified image lightvalue B*_(L) and the modified image dark value B*_(T) then deriveaccording to the following equations taking the standard deviation ofthe image light and image dark values into consideration:

B* _(L) =B _(LM) −D×G

B* _(T) =B _(TM) +D×G

The analysis values for the individual video sequences automaticallydetermined on the basis of the video signal values L, a and b of the L,a b color can be manually modified as needed by the operator in order,for example, to achieve special color designs. The automaticallydetermined or, potentially, manually modified analysis values aresupplied to a correction computer 7. In the correction computer 7,corresponding three-dimensional correction functions L_(KOR), a_(KOR),b_(KOR)=f(L, a, b) for the individual video sequences, for example ICCcolor profiles, are calculated on the basis of the analysis values ofthe individual video sequences and are input into a correction unit 8.By observing the individual scenes of the video film on a monitor, forexample, the operator can thereby decide whether each video sequenceshould be corrected with its separately calculated ICC color profile orwhether a plurality of successively following video sequences should becorrected with the ICC color profile of one of the successivelyfollowing video sequences.

The color correction occurs after the calculation of the ICC colorprofiles. For the purpose, the video signal values R, G and B are firstsequentially read out from the input image store 2 and are transformedinto the video signal values (L, a and b) in the input color converter5. The transformed video signal values L, a and b are then corrected inthe correction unit 8 according to the correction function L_(KOR),a_(KOR), b_(KOR)=f(L, a b). Subsequently, the corrected video signalvalues L_(KOR), a_(KOR) and b_(KOR) are transformed back into thecorrected video signal values R_(KOR), G_(KOR) and B_(KOR) in an outputcolor converter 9 according to the function R_(KOR), G_(KOR),B_(KOR)=f(L_(KOR), _(Akor), b_(KOR)) and are intermediately stored in anoutput image store 10 until further-processing.

The input color converter 5, the correction unit 8 and the output colorconverter 9 are formed, for example, as lookup tables (LUT) wherein theoutput values are stored in addressable fashion by the functionallycorresponding input values.

Although various minor changes and modifications might be proposed bythose skilled in the art, it will be understood that my wish is toinclude within the claims of the patent warranted hereon all suchchanges and modifications as reasonably come within my contribution tothe art.

What is claimed is:
 1. A method for image analysis and correction ofvideo signal values in processing of a video film composed of videosequences, each video sequence being composed of a plurality of frames,comprising the steps of: transforming video signal values of a firstcolor space according to a three-dimensional conversion function intovideo signal values of a second color space that is independent of thefirst color space and is equidistant and conforming to sensation;forming at least one analysis value representative of a video sequenceby forming an average of analysis values of analyzed frames of the videosequence; respectively determining a three-dimensional correctionfunction from the analysis values representative of the video sequence;and correcting at least the video signal values of the correspondingvideo sequence according to the identified correction function.
 2. Themethod according to claim 1 wherein all frames of the video sequence areutilized for the image analysis of the video sequence; and at least oneanalysis value representative of the video sequence is formed by formingan average of the analysis values of the frames of the video sequence.3. The method according to claim 1 wherein frames of the video sequenceare selected for the image analysis; and at least one analysis valuerepresentative of the video sequence is formed by forming an average ofthe analysis values of the selected frames of the video sequence.
 4. Themethod according to claim 1 wherein the correction function is an ICCcolor profile.
 5. The method according to claim 1 wherein in theanalysis of the image range of a frame, the frequency distribution ofthe brightness component of the transformed video signal values isidentified in the frame; and as analysis values, an image light value ofthe frame and an image dark value of the frame are determined from thefrequency distribution by defining maximum and minimum brightness valuesdependent on the curve of the frequency distribution in an image lightregion and an image dark region.
 6. The method according to claim 1wherein in the analysis of the image gradation of a frame, the frame tobe analyzed is geometrically divided into a plurality of sub-images; afrequency distribution of brightness components of the video signalvalues in the corresponding sub-image is defined as a sub-imagehistogram for each sub-image; the sub-image histograms of the individualsub-images are interpreted and the sub-images that are image-importantfor the image gradation are identified on the basis of the evaluation;an aggregate histogram is calculated from the sub-image histograms ofthe image-important sub-images, said aggregate histogram correspondingto the brightness distribution of the brightness component of the imagevalues in the image-important sub-images; and a correction curve forcorrection of an image gradation characteristic of the frame isdetermined as an analysis value from the aggregate histogram accordingto a method of histogram modification, which is determined for thepurpose of contrast correction.
 7. The method according to claim 1wherein in the analysis of a color cast of a frame, for acquiring acolor cast in the frame, dividing a region of high brightness values anda region of low brightness values of the brightness component of thevideo signal values in the frame to be analyzed into at least onebrightness range; determining a value of a potentially existing colorcast in the respective brightness range by averaging the colorcomponents of the video signal values in the brightness ranges; andforming the color cast value for the evaluation of the color castanalysis as an analysis value by selecting or combining color castvalues determined for the individual brightness ranges.
 8. A method forimage analysis and correction of video signal values in processing of avideo film composed of video sequences, each video sequence beingcomposed of a plurality of frames, comprising the steps of: transformingvideo signal values of a first color space according to athree-dimensional conversion function into video signal values of asecond color space different than the first color space and conformingto sensation; forming at least one analysis value representative of avideo sequence by forming a representative value of analysis values ofanalyzed frames of the video sequence; respectively determining athree-dimensional correction function from the analysis valuesrepresentative of the video sequence; and correcting at least the videosignal values of the corresponding video sequence according to theidentified correction function.